There is a particular desire for synchronous motors used in compressors, electric cars, hybrid cars, fuel cell-powered cars, and the like to produce high torque with low torque ripple, given the demand for motors that are small, lightweight, high-output, low-vibration, low-noise, and efficient.
In a surface magnet type synchronous motor, in which permanent magnets are disposed on the surface of a rotor core, the torque produced by the permanent magnet (magnetic torque) is at a maximum when the magnetic field produced by the permanent magnets and the armature current differ in phase by 90°, i.e. when the inter-polar gaps on the rotor and the stator teeth around which stator coils are wound oppose each other so that the current supplied to the stator coils is at a maximum. Any deviation from the 90° phase difference between the permanent magnet-produced magnetic fields and the armature current results in reduced torque.
Also, in an interior permanent magnet synchronous motor, in which the permanent magnets are arranged inside the core, in addition to magnetic torque from the permanent magnets, reluctance torque is also produced due to the salient polarity owing to the difference in magnetic reluctance caused by the positions of the rotor and stator. Reluctance torque is at a maximum when the phase difference between the permanent magnet-produced magnetic fields and the armature current is approximately 45°. Accordingly, the torque from an interior permanent magnet synchronous motor is a combination of magnetic torque and reluctance torque, and that torque is at a maximum when the phase difference between the magnetic fields and armature current is between 0° and approximately 45°.
Ordinarily, the torque of a synchronous motor includes a ripple component that is based on the influence of the harmonic component of the permanent magnet-produced magnetic fields, the influence of the harmonic component of the armature current, and the like. To address this issue, there exists technology for reducing torque ripple by mechanically offsetting the placement interval (angle) of the stator coils, through which flows current in a single phase, from the inter-polar gap (angle) of the rotor. Through the use of such technology, the phases of the torque ripple produced by the stator coils are offset from each other and the torque ripple can be negated. As a result, a low-vibration, low-noise motor can be achieved. The following documents disclose technology for achieving low-vibration and low-noise motors.
Patent Literature 1 discloses a motor provided with inner and outer teeth on an annular yoke, a plurality of coils toroidally wound around the teeth, an inner rotor corresponding to the inner teeth, and an outer rotor corresponding to the outer teeth, wherein the point at which poles of the outer rotor and of the inner rotor change are offset by any angle when the rotors are attached, thus achieving low vibration.
Patent Literature 2 discloses setting the magnetic salient pole of the outer rotor and the magnetic salient pole of the inner rotor to the same position in the circumferential direction, resulting in the radial components of the electromagnetic force between the magnetic salient poles of the outer and inner stator teeth offsetting each other, thereby decreasing vibration due to cycle variation.